Replace deprecated pydantic-ai MCP classes with MCPToolset in common.ai

[kaxil]

pydantic-ai deprecated MCPServerStdio, MCPServerSSE, and MCPServerStreamableHTTP in favor of a unified MCPToolset, and removes all three in pydantic-ai v2. The common.ai MCPHook constructed all three directly, so it emits DeprecationWarnings today and would break on the v2 upgrade.

This migrates MCPHook.get_conn() to build an MCPToolset over the matching FastMCP transport.

What changed

Transport	Before	After
http	MCPServerStreamableHTTP(host, headers=…, tool_prefix=…)	MCPToolset(StreamableHttpTransport(host, headers=…))
sse	MCPServerSSE(host, headers=…, tool_prefix=…)	MCPToolset(SSETransport(host, headers=…))
stdio	MCPServerStdio(cmd, args=…, timeout=…, tool_prefix=…)	MCPToolset(StdioTransport(command=cmd, args=…), init_timeout=…)
prefix	tool_prefix= constructor arg	.prefixed(prefix)

Design rationale

• Explicit transports instead of URL inference. MCPToolset can infer a transport from a bare URL, but FastMCP only picks SSE when the URL ends in /sse. The connection has an explicit transport field, so the hook builds the transport explicitly to honor that choice regardless of URL shape, and to keep attaching the Authorization header.
• StdioTransport for stdio. The connection supplies an arbitrary command + args, which is the case pydantic-ai's deprecation message prescribes StdioTransport(...) for; the MCPToolset("script.py") shorthand only covers literal script paths.
• .prefixed() for tool_prefix. MCPToolset has no tool_prefix argument. .prefixed(prefix) produces the same <prefix>_<tool> names as before and matches pydantic-ai's own load_mcp_toolsets. It returns a PrefixedToolset that proxies the async-context-manager protocol, so the connection-backed MCPToolset wrapper keeps working unchanged.
• init_timeout is the successor to the old stdio timeout; the hook keeps its default of 10s.

Tradeoffs / notes

• Behavior-preserving: tool names, auth headers, caching defaults, and the no-auth path are unchanged.
• fastmcp.client.transports is the canonical public import path. pydantic_ai.mcp imports from it, and pydantic-ai's own deprecation message recommends MCPToolset(fastmcp.client.transports.StdioTransport(...)).

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[gopidesupavan]

LGTM thanks kaxil :)

not a blocker can we do autospec for tests ?
@patch(_MCP_TOOLSET, autospec=True)
@patch(_HTTP_TRANSPORT, autospec=True)

[kaxil]

Thanks @gopidesupavan! Good call -- done in 42c8b27. All the @patch decorators (and the mask_secret patch) now use autospec=True.